The structure of such aircraft needs to be compatible with standardized functional constraints, and also with certification, in particular.
In the event of the aircraft impacting against the ground (crashing), structural damage must be limited and even prevented, in particular in the vicinity of the cabin and the vital components of the aircraft. All unacceptable injury to humans must be avoided.
At present, complying with certain specifications is in contradiction with obtaining functions that are desired for such and such a component of the aircraft, thus requiring compromises to be made that are not always acceptable in practice. This applies in particular to rotary wing aircraft.
The search for increasing the capacity of the structure of an aircraft to absorb energy due to an impact (accident) has lead to numerous proposals.
Patent FR-2 632 604 describes a helicopter structure comprising a frame that is essentially constituted by a gantry whose ends are connected together by a cross-member. A particular structure makes it possible to obtain controlled deformation of the cross-member by initiating buckling in the event of an impact.
U.S. Pat. No. 4,084,029, U.S. Pat. No. 4,734,146, and FR-2 817 608 relate to beams having a composite corrugated core that undulates with a sinusoidal shape.
Various other structures for absorbing energy in the event of an impact to the fuselage of an aircraft against the ground have also been proposed.
U.S. Pat. No. 6,620,484 describes an absorption structure comprising vertical composite panels having a thread stitched therein at a density that increases from the bottom of the structure towards its top portion.
U.S. Pat. No. 4,593,870 relates to an impact absorber that extends under the floor of a helicopter cabin and that comprises a crossed array of beams including a laminate or two laminates covering a honeycomb-structure core. Rupture-initiators are provided in the bottom of the laminate.
U.S. Pat. No. 4,941,767 describes intersecting planes with connections via ring-section parts.
U.S. Pat. No. 5,069,318 describes reinforcement surrounding a thin wall in order to stabilize its behavior in the event of a crash. U.S. Pat. No. 6,718,713 describes a preshaped member with a channel for inserting a plane partition. Document WO 03/018295 describes a member that is preshaped to the shape of the Greek letter “Pi”, for assembling together composite-material parts.
The present invention applies in particular to rotorcraft in which the bottom structure includes a compartment that receives, or that is to receive, equipment (such as a fuel tank) that is liable, in the event of a crash, to exert thrust against a side wall of the compartment.
Patent FR-2 763 313 describes an installation for suspending a flexible fuel tank by means of straps within a compartment. In the event of a crash, the kinetic energy due to the mass of fuel causes the bottom partition of the compartment to rupture and subjects the straps to plastic deformation.
U.S. Pat. No. 5,451,015 describes a helicopter having a structure that includes a compartment receiving a flexible fuel tank; the compartment comprises four side walls and a bottom wall that are all reinforced by stiffeners. Polyurethane foam is provided between the flexible tank and said walls. The respective strengths of the foam and of the walls are adjusted such that in the event of an impact, the thrust from the fuel does not completely destroy the walls of the compartment.